Quantum dots are semiconductor nanocrystals ranging from nanometers in size to a few microns. The size controls the number of electrons contained in the dot. Each quantum dot can contain from one to several thousand electrons. Since quantum dots are so small, quantum mechanical effects force the electron energy levels to be quantized. Quantum dots have sometimes been called artificial atoms because the electron quantum levels contained within a dot are similar to the electron orbitals in an atom.
This quantization allows distinct wavelength (colors) of light to be emitted. Light or electric current typically excites them. The light emitted ranges from ultraviolet to visible to infrared, depending on the material and the size. This is a wavelength span of 350 to 2300 nanometers. The emission has a very narrow bandwidth of 30 nanometers, full width at half maximum (FWHM). Work is being conducted on using quantum dots as a replacement for LEDs.
In addition to semiconductors, quantum dots can be made from metal. Presently, quantum dots are made by vacuum techniques such as molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), or in aqueous solutions where a colloid is formed. Other techniques may be developed. Cadmium selenide (CdSe) is a common material for visible light quantum dots. A 3 nanometer CdSe dot emits 520-nanometer light that is green. Increasing the diameter to 5.5 nanometers increases the wavelength to 630 nanometers, the wavelength of red light. Quantum dots can be “tuned” by controlling their size to have any desired pure color of a desired wavelength. Other phosphors suitable for quantum dots include doped zinc sulfide (ZnS) compounds. Gold quantum dots have also been made. CdSe quantum dots may be coated with ZnS as a protective layer.
Quantum dots are commercially available from companies such as Evident Technologies of Troy, N.Y. Quantum dots have a myriad of applications, including medical applications for tagging proteins and antibodies. The quantum dots fluoresce to map the proteins and antibodies. Other uses of quantum dots include photovoltaic solar cells, electroluminescent devices, the phosphorous of LED lights, thermoelectrics, inks, pigments and anti-counterfeiting materials, to mention just a few areas of current research and development.
Quantum dots have been proven to be radiation resistant. It would therefore be advantageous if energy, namely alpha or beta particles from nuclear sources, could be utilized as the energy source to energize quantum dots for use in light sources having precise wavelengths and intensities, which precise light sources could be used, for example, to calibrate light sources. Other uses of these quantum dots include their uses in calibrating detector equipment such as ATP luminometers used for measuring the presence of ATP in swab samples, etc.